Carrier aggregation is a combination of two or more cells or component carriers (CCs) operating at different frequencies in order to provide a broader transmission bandwidth for a mobile terminal. Depending upon its capabilities, a mobile terminal may simultaneously receive or transmit on one or more of the cells. The cells aggregated in accordance with carrier aggregation include a primary cell and one or more secondary cells. The primary cell is the cell that: (i) operates on a primary carrier in which the mobile terminal either performs the initial connection establishment procedure or initiates the connection re-establishment procedure, or (ii) was indicated as the primary cell in a handover procedure. Conversely, a secondary cell is a cell, operating on a secondary carrier, which may be configured once radio resource control (RRC) is established and which may be used to provide additional radio resources.
Although the focus to date has been principally upon frequency division duplex (FDD) networks, time division duplex (TDD) networks that support carrier aggregation must also be considered. Indeed, in a TDD network, the primary cell and the secondary cells may have respective TDD uplink (UL)/downlink (DL) subframe configurations. In Long Term Evolution (LTE) release 10, mobile terminals that support TDD signaling are required to operate in accordance with a TDD UL/DL subframe configuration that is aligned and consistent across the primary and secondary cells that are to be aggregated. Additionally, LTE release 10 required that common discontinuous reception (DRX) parameters be utilized for each of the primary and secondary cells such that the active time and the DRX pattern would be the same for each of the aggregated cells.
For mobile terminals configured in accordance with LTE release 11, however, the primary and secondary cells are permitted to have different TDD UL/DL subframe configurations. These different TDD UL/DL subframe configurations may be useful in order to allow a cell to be compatible with a neighbor legacy TDD system, such as a Time Division Synchronous Code Division Multiple Access (TD-SCDMA) system, for which multiple cells having aligned TDD UL/DL subframe configurations are not available. Additionally, primary and secondary cells having different TDD UL/DL subframe configurations may provide different amounts of resources and a different DL/UL ratio for the mobile terminal. For example, the use of different TDD UL/DL subframe configurations by the primary and secondary cells in LTE release 11 may permit different ones of the cells to provide different coverage by, for example, enabling more UL subframes in a lower frequency cell to enlarge the coverage. The ability to provide different amounts of resources and a different DL/UL ratio may be of particular importance to a mobile terminal that utilizes carrier aggregation since carrier aggregation is oftentimes utilized in an instance in which the mobile terminal has a relatively large amount of data to transmit, thereby increasing the importance of defining the TDD UL/DL subframe configurations of the primary and secondary cells so as to provide a suitable DL/UL ratio and to otherwise efficiently utilize the communication resources.
By allowing the primary and secondary cells to have different TDD UL/DL subframe configurations, however, the different TDD UL/DL subframe configurations of the primary and secondary cells may have overlapped subframes in some instances, such as by one of the cells having a DL subframe at a specific instance in time while another cell has an UL subframe. An example of the TDD UL/DL subframe configuration of a primary cell and a secondary cell is shown in FIG. 1. In this example, subframe 3 is overlapping in that subframe 3 of the primary cell is a UL subframe, while subframe 3 of the secondary cell is a DL subframe.
The mobile terminal could be configured to provide for simultaneous reception and transmission so as to accommodate overlapping subframes. While the simultaneous transmission and reception would permit all subframes to be utilized and would avoid missing any transmission and reception opportunities, a mobile terminal having simultaneous transmission and reception capabilities for TDD signaling would require a duplex filter which would disadvantageously add to the complexity of the TDD implementation of the mobile terminal. As an alternative to configuring the mobile terminal to allow for simultaneous transmission and reception, the mobile terminal could, instead, only process one of the overlapping subframes with the other of the overlapping subframes being blocked or otherwise ignored.
The overlapping subframes may also increase the complexity with which various DRX parameters are defined. In this regard, DRX parameters, such as the on-duration timer, the inactivity timer and the retransmission timer, are defined in terms of the number of Physical Downlink Control Channel (PDCCH) subframes. In this regard, a PDCCH subframe in a TDD system is defined in LTE releases 8, 9 and 10 as either a DL subframe, e.g., the subframes designated D in FIG. 1, or a special subframe, e.g., the subframes designated S in FIG. 1. In an instance in which the TDD UL/DL subframe configurations of the primary and secondary cells have overlapping subframes, however, the DRX parameters that are defined in terms of a number of PDCCH subframe may be somewhat unclear since the primary and secondary cells may have different numbers of PDCCH subframes as a result of the overlapping subframes. By way of example, a PDCCH subframe could be defined as a subframe having at least one DL subframe or special subframe in any of the corresponding subframes, e.g., the subframes aligned in time, of the primary and secondary cells. Unfortunately, this definition of a PDCCH subframe may create an imbalance in the monitoring of the different cells and create additional complexity for the base station which may find it difficult to control the DRX pattern.
As shown in FIG. 2, for example, an on-duration timer may be defined to equal 4 PDCCH subframes (PSF). Based upon the definition of a PDCCH subframe as a subframe that is either a DL subframe or a special subframe in any of the corresponding subframes of the primary and secondary cells, the mobile terminal may maintain an active state for the first 5 subframes (including 4 PDDCH subframes and an UL subframe) and may then become inactive. As such, a downlink grant for the primary cell that arrived during subframe 6 will not be acted upon since the mobile terminal is in an inactive state even though the TDD UL/DL subframe configuration of the primary cell includes only 2 PDCCH subframes, namely, 1 DL subframe and 1 S subframe, during the active time period. As such, the mobile terminal in this example would not have the same chance to capture the scheduling grant on each of the primary and secondary cells, which impairs one of the objectives of carrier aggregation, namely, load balancing.
Additionally, the definition of the PDCCH subframe as a subframe that is either a DL subframe or a special subframe in the TDD UL/DL subframe configurations for any of the primary and secondary cells may also cause the different cells to have different numbers of retransmission opportunities as shown in FIG. 3. By way of example, a retransmission timer (Rx timer) may have a value of 2 PDCCH subframes following a Hybrid Automatic Request Round Trip Time (as denoted by the RTT timer of FIG. 3) of 7 PDCCH subframes. As shown in FIG. 3, the retransmission period may last for only two subframes since the secondary cell includes 2 DL subframes following the start of the retransmission timer. Thus, the mobile terminal of this example that may be unable to detect a retransmission on the primary cell that occurs during the subframe immediately following expiration of retransmission period even though the primary cell included UL subframes during the retransmission period.
In addition to issues relating to the manner in which PDCCH subframes are defined, overlapping subframes may create additional challenges. For example, in an instance in which a DL subframe of the primary cell overlaps with a UL subframe of a secondary cell, the mobile terminal may be configured to process the UL subframe of the secondary cell in accordance with the TDD UL/DL subframe configuration of the primary cell, thereby processing the UL subframe of the secondary cell as a DL subframe and leading to unnecessary blind decoding to detect downlink control channels and unnecessarily consuming power of the mobile terminal. Alternatively, in an instance in which the overlapping subframes include a DL subframe in the primary cell and a corresponding UL subframe in the secondary cell, the avoidance of the UL subframe in the secondary cell may also disadvantageously block the DL subframe in the primary cell.
By way of example, reference is now made to FIG. 4 in which the primary and secondary cells have TDD UL/DL subframe configurations #0 and #2, respectively, which, in turn, have overlapped subframes in subframes 3, 4, 8 and 9. As shown, the primary cell has UL subframe and the secondary cell has DL subframe for the overlapping subframes. In this example, the TDD UL/DL subframe configuration of the primary cell is assumed for the secondary cell so as to block overlapping subframes in the secondary cell. As shown in FIG. 4, the DL subframes of the secondary cell that overlapped with UL subframes of the primary cell may be blocked by scheduling restrictions overlapping with the UL subframes of the primary cell. In this regard, the mobile terminal will not monitor the PDCCH or use Cell-specific Common Reference symbols (CRS) for measurements since the mobile terminal will assume that the overlapping subframes of the secondary cell are UL subframes based upon the TDD UL/DL subframe configuration of the primary cell. Thus, the mobile terminal will not receive DL scheduling grants for the secondary cell during the overlapping subframes.
In another example illustrated in FIG. 5, the overlapping subframes may include DL subframes for the primary cell and UL subframes for the secondary cell. In this example, the overlapping UL subframes of the secondary cell may be blocked via DRX overlapping with the corresponding DL subframes of the primary cell. In this example, discontinuous reception may eliminate the need for the mobile terminal to monitor the PDCCH as the mobile terminal may assume that discontinuous reception is configured for the DL subframes based upon the TDD UL/DL subframe configuration of the primary cell. Thus, the overlapping DL subframes in the primary cell may not be utilized.